ak-47 is amazing for reliability.  If you want something that's going to be cheap to stock assloads of ammo for, get anything that fires a 5.56mm full jacket - you can get cases of 5,000 rounds for surprisingly cheap.

but this... this is my ultimate love...  this is the opposite end of the spectrum (and obviously for a bit closer range) i.e. NOT CHEAP.  In fact, you can buy a fairly nice car for less than one of these.

https://www.youtube.com/watch?NR=1&v=WOoUVeyaY_8&feature=fvwp